linux-kernel - Re: [PATCH] sched: balance_cpu to consider other cpus in its group as target of (pinned) task migration

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Date:	Mon, 04 Jun 2012 16:30:34 +0200
From:	Mike Galbraith <efault@....de>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Prashanth Nageshappa <prashanth@...ux.vnet.ibm.com>,
	mingo@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	roland@...nel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: balance_cpu to consider other cpus in its group
 as target of (pinned) task migration

On Mon, 2012-06-04 at 18:37 +0530, Srivatsa Vaddagiri wrote: 
> * Mike Galbraith <efault@....de> [2012-06-04 14:47:43]:
> 
> > You need a good reason to run RT, and being able to starve others to
> > death ain't it, so I don't see a good reason to care about the 95% case
> > enough to fiddle with load balancing to accommodate the oddball case.
> 
> While starvation of SCHED_OTHER task was an extreme case, the point
> remains that SCHED_OTHER tasks are better served by moving them away
> from cpus running rt tasks that are partially cpu intensive. While the
> current code has the nuts and bolts to recognize this situation
> (scale_rt_power), it fails to effect SCHED_OTHER task movement because of how
> one cpu from a sched_group is designated to pull tasks on behalf of its
> siblings and that chosen balance_cpu may not be in the task's cpus_allowed mask
> (but the task can run on one or more of its sibling cpus).

Yeah, this is true, it is a latency source and a fairness violation.
Slow path balance consideration does make some sense to me.

But, if you have an RT requirement, you can't afford to mix unknown
entities, nor over-commit etc.  A realtime application will assign all
resources, so the load balancer becomes essentially unemployed.  No?
Non critical worker-bees may be allowed to bounce around in say a
cpuset, but none of the CPUs which do critical work will ever be
over-committed, else application just lost the war.  In that regard,
twiddling the load balancer to accommodate strange sounding case still
seems wrong to me.

-Mike

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